Method and apparatus for evaluating surface and subsurface features in a semiconductor

ABSTRACT

A method and apparatus are disclosed for evaluating surface and subsurface features in a semiconductor sample. In operation, a periodic energy source is applied to the surface of the semiconductor sample to generate a periodic electron-hole plasma. This plasma interacts with features in the sample as it diffuses. The plasma affects the index of refraction of the sample and the changing plasma density is monitored using a radiation probe. In the preferred embodiment, the radiation probe measures the plasma induced periodic changes of reflectivity of the surface of the sample to yield information about the sample, such as ion dopant concentrations, residue deposits and defects.

This is a continuation of application Ser. No. 076,876, filed July 23,1987, now abandoned, which is in turn a continuation of application Ser.No. 707,485, filed Mar. 1, 1985.

DESCRIPTION

1. Technical Field

The subject invention relates to a new and improved method and apparatusfor evaluating surface and subsurface features in a semiconductor.Information is derived by analyzing the interaction between samplefeatures and an electron-hole plasma induced in the sample. Variationsin plasma density, which is in part, a function of variations in thesample, are measured based on the effect of the plasma on the refractiveindex at the surface of the sample. A radiation probe is reflected offthe surface of the sample and changes induced in the radiation probe bythe plasma-induced changes in the refractive index are monitored toobtain information about surface and subsurface characteristics of thesample.

2. Background of the Invention

There has been considerable effort expended in developing tools for thenondestructive analysis of materials. This interest is particularlystrong in the integrated circuit industry. In the prior art, techniqueswere developed where high powered optical microscopes are used toanalyze surface characteristics of a sample. Subsurface features havebeen analyzed through the use of acoustic waves that are generated inthe sample and interact with the elastic features beneath the surface ofthe sample. More recently, a new branch of investigations has developedwherein thermal waves are used to derive information about thermalfeatures in a sample.

In a thermal wave system, a localized periodic heating is induced at thesurface of the sample. Energy from the heat source is absorbed by thesample at or near its surface and a periodic surface heating occurs atthe modulation frequency of the heat source. This periodic surfaceheating is the source of thermal waves that propagate from the heatedregion. The thermal waves interact with thermal boundaries and barriersin a manner that is mathematically equivalent to scattering andreflection of conventional propagating waves. Thus, any features on orbeneath the surface of the sample that have thermal characteristicsdifferent from their surroundings will reflect and scatter thermal wavesand thus become visible to these thermal waves. Thermal waves can beinduced in a wide variety of sample materials and can be used to detectthese thermal features. Thermal waves, however, are critically dampedand travel only about one thermal wavelength thus the penetration rangeis quite limited.

The subject invention is directed to a new and improved nondestructiveanalytical tool which in some respects is quite analogous to systemswhich perform thermal wave analyses. In the subject invention, thedensity variations of a diffusing electron-hole plasma are monitored toyield information about features in a semiconductor.

As is well known, semiconductors have a band gap between the valence andconduction bands. Input energy is needed to raise an electron from thevalence band to the conduction band which results in the creation of anelectron-hole pair. Typically the input energy to the system will exceedthe band gap energy and the electron will be excited from the valanceband to an energy level above the conduction band. These electroncarriers will, in a relatively short period of time (τ≅10⁻¹³ seconds),give up a portion of their energy to the lattice through nonradiativetransitions to the unoccupied states near the bottom of the conductionband. After a much longer time (τ=10⁻³ to 10⁻⁸ seconds) these carrierswill give up the remainder of their energy to the lattice by recombiningwith the holes of the valence band. Prior to this recombination, thereexists a plasma of electrons and holes whose spatial density is governedby diffusion in a manner analogous to the flow of heat from a thermalsource.

If an evaluation is made of this plasma diffusion, information can bederived about the composition and lattice structure of a semiconductor.In some situations, where the plasma is generated in a periodic fashion,"plasma waves" can be generated and information about subsurfacefeatures can be derived using an analysis similar to a thermal waveanalysis.

Changes in plasma density will result in a change in the index ofrefraction at the surface of a sample This dependance has been reportedby D. H. Auston et al., in "Picosecond Ellipsometry of TransientElectron-Hole Plasmas in Germanium" (Physical Review Letters, Vol 32,No. 20, May 20, 1974). This paper reports that changes in the index ofrefraction, due to the variations in plasma density, can be detected byreflecting a probe beam off the surface of the sample within the areawhich has been excited. (See also "Picosecond Time-Resolved Plasma andTemperature-Induced Changes of Reflectivity and Transmission inSilicon," J. M. Liu, et. al., Applied Physics Letters Vol. 41 No. 7Oct.1, 1982). These preliminary articles were merely attempting toanalyze how the plasma moves through a sample. No effort was made toanalyze the sample itself through the interaction of the plasma with thesample. Furthermore, the energy source was not modulated, that is, aperiodic plasma was not generated, and thus would prevent an analysissimilar to that described in the subject invention.

When the energy source is modulated and a periodic plasma is generated,the probe beam, which is reflected off the surface of the sample, willundergo periodic changes in both intensity and phase. Changes inintensity can be measured by a relatively simple photodetectortechnique. Changes in phase can be measured through monitoring byinterferometry techniques or by monitoring the periodic angulardisplacements of a probe beam.

Very recently, some attempts have been made to analyze the plasmathrough its affects on acoustic waves generated in a silicon sample.(See "Effect of Photocarriers on Acoustic Wave Propagation for MeasuringExcess Carrier Density and Lifetimes in Silicon", Stearns, et al.,Applied Physics Letters, Vol. 45 No. 11, Dec. 1, 1984). In theexperimental arrangement reported in this article, the energy source wasmodulated and a periodic plasma was generated. However, there was noattempt made to analyze the sample itself through interactions of theplasma with the sample. Furthermore, the analytical tool described inthe latter article was a contact technique requiring an acoustictransducer.

In order to detect optical changes in the index of refraction it isnecessary that the probe beam be located within the periodically excitedarea. The periodically excited area can be defined in terms of a radiuswith the center point being the center of the energy source as follows:##EQU1## where r_(o) is the radius of the energy source and u is thedistance over which the plasma will diffuse in the sample. In thesituation where the decay time τ (the time it takes for theelectron-hole pairs to recombine) is relatively short compared to themodulation period 1/ω, where ω is the modulation frequency inradians/second, (ie, ωτ is less than 1). then the diffusion length (u)of the plasma is given by the following equation:

    U=(Dτ).sup.1/2

where D is the diffusivity of the plasma.

A more interesting situation occurs when the decay time τ is longcompared to the period of the modulation of the energy beam (i.e., ωτ isgreater than 1) In this case, "plasma waves" will be created and u isgiven by the following equation:

    u=(2D/ω).sup.1/2

These "plasma waves" are critically damped and can be analyzed in amanner directly analogous to thermal waves. More specifically, in thislimiting case, the plasma diffusion length u depends on the modulationfrequency ω, and can therefore be varied by changing the modulationfrequency. Information about the subsurface region as a function ofdepth beneath the sample surface is obtained by studying the periodicchanges in the probe beam when the modulation frequency of the energysource is varied. This analysis is directly analogous to the studiesdescribed in detail in copending U.S. Patent Application Ser. No.389,623 filed on June 18, 1982, and now U.S. Pat. No. 4,513,384, issuedApr. 23, 1985 assigned to the same assignee as the subject invention andincorporated herein by reference.

The analysis described in the latter patent application is intended togive information as to either layer thickness or compositional variablesof a sample as a function of depth. These techniques can be applied withthe method of the subject invention when the decay time τ is longcompared to the period of modulation of the energy beam.

As described above, there are many important and significantsimilarities between thermal wave analysis and plasma density analysis,which is the subject of this application. There are also importantdifferences. Most importantly, electron-hole plasma analysis is limitedto semiconductor materials. However, when semiconductor analysis isdesired, this technique provides some advantages over a thermal waveanalysis. For example, plasma density analysis can be significantly moresensitive than a thermal wave analysis. Thermal wave studies onlyprovide information as to thermal features. Plasma density analysis,which can be thought of as an analysis of the movement of highlyinteractive electrons, will provide information on a wide variety ofchanges in the structure and composition of a semiconductor sample.Furthermore, experiments have shown that the sensitivity of the plasmato variations in some sample characteristics can be anywhere from 10 to100 times greater than that which would be expected from a thermal waveinteraction alone.

Another distinguishing feature of this system is that unlike a thermalwave approach, a periodic heat source is not required. As pointed outabove, in order to do a thermal wave analysis, it is necessary to inducea periodic localized heating on the sample surface to generate thermalwaves. In the subject system, all that is required is a periodic energysource which will interact and excite electrons from the valance band tothe conduction band. In practice, the means for exciting the plasma willbe similar to those commonly used in generating thermal waves. However,it should be understood that the generation of heat is not required, andthat it is only necessary to impart enough energy to the electrons toovercome the band gap in the sample. If this energy is carefullycontrolled, no localized heating will occur.

Another difference between the subject approach and thermal wave systemsis that despite the fact that the energy beam is modulated, a "plasmawave" will not always be generated. As pointed out above, if the periodof the modulated energy beam is greater than the recombinant or decaytime (τ), no plasma waves will be created. In contrast, if themodulation frequency (ω) is controlled such that the period betweencycles is less than the decay time (τ), plasma waves will be created.

For many measurement situations, a wavelike phenomenon, such as theplasma wave, is unnecessary for evaluation. For example, and asdiscussed in detail in the specification, applications which do notrequire depth profiling or the analysis of sample variations as afunction of depth, do not require the generation of plasma waves.However, in applications where sample variations as a function of depthneed to be studied, it is necessary to generate and study plasma waves.

Therefore, it is an object of the subject invention to provide a new andimproved method and apparatus for evaluating surface and subsurfaceconditions in a semiconductor sample.

It is a further object of the subject invention to provide a new andimproved method and apparatus for analyzing semiconductors wherein asample is excited in a manner to create an electron-hole plasma.

It is another object of the subject invention to provide a method andapparatus wherein features in a semiconductor are analyzed by generatingan electron-hole plasma in the sample and monitoring the diffusion ofthis plasma using a radiation probe.

It is a further object of the subject invention to provide a method andapparatus for evaluating surface and subsurface conditions in asemiconductor sample wherein changes in a reflected probe beam aremonitored to study the density variations of the electron-hole plasma.

SUMMARY OF THE INVENTION

In accordance with these and many other objects, the subject inventionprovides for a new and improved apparatus and method for evaluatingsurface and subsurface conditions in a semiconductor sample. Morespecifically, the subject invention is intended to permit nondestructiveanalysis of various properties of semiconductors. As discussed ingreater detail below, the subject system is capable of detectingresidues and defects, measuring ion dopant concentrations, and in thepreferred embodiment, can also yield information relating to layerthicknesses and compositional variables as a function of depth.

In accordance with the subject invention, a means is provided forperiodically exciting electrons in the sample to create electron-holepairs or a plasma. The periodic energy can be supplied from a widevariety of sources including particle beams and chopped or modulatedelectro-magnetic radiation beams. In the preferred embodiment, anintensity-modulated laser beam is utilized.

As discussed above, the infusion of energy will result in the creationof an electron-hole plasma which propagates away from the energy source.The plasma will interact with surface and subsurface features of thesample such that the diffusion profile of the plasma will be altered.The diffusion profile or changing density of the plasma is detected atthe sample surface through the use of a radiation probe.

In accordance with the subject invention, the radiation probe isdirected to the surface of the sample such that it falls within at leasta portion of the area which is being periodically excited. The periodicchanges in the plasma density will affect the index of refraction of thesample and thus cause periodic changes in the reflected radiation probe.These changes are then monitored and processed to evaluate the sample.

The radiation probe will undergo changes in both intensity and phase. Inthe preferred embodiment, the changes in intensity, caused by changes inreflectivity of the sample, are monitored using a photodetector. It ispossible to detect changes in phase through interferometric techniquesor by monitoring the periodic angular deflections of the probe beam.

Further analytical information can be obtained where the system isarranged to generate "plasma waves". More specifically, if the periodbetween cycles of the energy beam is less than the decay time of theplasma, critically damped plasma waves will be created. These plasmawaves may be treated analytically in a manner similar to criticallydamped thermal waves. A more complete mathematical presentation ofplasma waves will be given below. While the use of thermal waves toevaluate surface and subsurface conditions is relatively new, asignificant body of information already exists concerning methods ofhandling these analyses. Since the actual analyses are not a part of thesubject invention, this disclosure will refer only to the portions thatare necessary for understanding of the subject application. However, theabove cited and following patents and patent applications should bereferred to if a complete background of thermal wave analysis isdesired. (See, Method For Detection of Thermal Waves With a Laser Probe,U.S. Ser. No. 401,511 filed July 26, 1982; and now U.S. Pat. No.4,521,118 issued June 4, 1985. Thin Film Thickness Measurements withThermal Waves, U.S. Ser. No. 481,275 filed Apr. 1, 1983; and now U.S.Pat. No. 4,552,510, issued June 11, 1985. Method and Apparatus ForEvaluating Surface Conditions of a Sample, Ser. No. 612,076, filed May21, 1984; and now U.S. Pat. No. 4,636,088 issued Jan. 13, 1087. andMethod & Apparatus for Detecting Thermal Waves, U.S. Ser. No. 612,075,filed May 21, 1984.) and now U.S. Pat. No. 4,579,463 issued Apr. 1,1986. All the cited patents and applications are assigned to the sameassignee as the subject invention and their disclosures are incorporatedherein by reference.

In a manner similar to that set forth in the above referenceddisclosures, and in particular, U.S. Pat. No. 4,513,384, where plasmawaves exists, additional information can be derived by varying themodulation frequency of the energy beam and observing changes in theobserved plasma density. By varying the frequency of the energy beam,information can be obtained as to subsurface features, permittinganalysis of layer thickness and composition variations as a function ofdepth. Further objects and advantages of the subject invention willbecome apparent from the following detailed description taken inconjunction with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an apparatus for performing thenew and improved analysis on semiconductors of the subject invention.

FIG. 2 is a schematic diagram, similar to FIG. 1, illustrating astructure wherein the changes in the probe beam can be monitored throughan interferometric technique.

FIG. 3 is a partial schematic diagram illustrating an alternativeembodiment wherein phase changes in the probe beam can be monitored bymeasuring the angular displacements of the probe beam.

BEST MODE FOR CARRYING OUT THE SUBJECT INVENTION

Turning to FIG. 1, there is illustrated a schematic diagram of anapparatus 20 which performs the method of the subject invention. Theparticular apparatus illustrated is particularly adapted for detectingresidues and defects and for microscopically evaluating ion dopantconcentrations in a semiconductor by measuring changes in reflectivityof a probe beam.

Background information concerning the problems associated with thedetection of residues and ion dopant concentrations is set forth ingreat detail in copending application, Ser. No. 612,076, cited above.Briefly, in many semiconductor manufacturing techniques, layers aresuccessively deposited on a substrate and thereafter etched to definethe desired circuitry. During many of these steps, it is possible thatthin layers of residues will remain on the sample surface.

For example, an oxide layer is often deposited and then etched using aphotoresist and masking technique during the fabrication of asemiconductor. In situations where neither the photoresist layer oroxide layer are fully removed, the integrated circuit will be prone tofailures. To date, the industry has not come up with any good methodsfor detecting these residues. In most manufacturing situations, theintegrated circuits are subjected to optical microscopic examination byhuman operators. As can be appreciated, this can be very time-consumingand inefficient. Furthermore, the thickness of the residues which canadversely affect the fabrication of the integrated circuits are so thinas to be nearly invisible, even when inspected through a microscope.Other techniques have been developed but they are not suitable forgiving information with regard to the presence of residues withinmicroscopically small regions.

Another technique in semiconductor manufacturing concerns theimplantation of ion dopants. These dopants are implanted to impartdifferent electrical conductivities to the substrate. In theimplantation process, an ion beam is rastered over the surface of thesample. Some of the ions in the beam which bombard the sample areintroduced into the lattice structure of the semiconductor. Theconcentration of the dopant ions in the sample is related to the lengthof time which the beam is focused on any point on the surface of thesample. The depth to which the ions penetrate is related to the voltageon the ion beam. However, even at a maximum voltage, the depth to whichthe ions penetrate is relatively small such that the ion concentrationsof interest are substantially at the upper surface of the sample.

The ions which are incorporated into the surface of the sample arelocated at interstitial sites and physically disrupt the latticestructure of the material. In this state, the material will not exhibitthe desired semiconductive properties. In order to overcome thisproblem, it is necessary to activate the dopant in a subsequentfabrication step. The dopant is activated through an annealing process.In this annealing process, the material is heated in a manner to permitthe lattice to reform, enabling the ions to move from the interstitialsites to substitutional sites. In this process, the dopant ions aresubstituted for substrate atoms at various points in the lattice. Thisannealing step functions to remove defects in the lattice and free theelectrons of the dopant ions for conduction of current.

Once the annealing step has been performed, the dopant levels which havebeen implanted usually can be measured by known electrical resistivitymethods. However, these methods are unable to detect concentrationsbelow ions per cm². The subject invention has been able to detectconcentrations as low as 10¹⁰ ions per cm². In addition, the subjectinvention is also capable of detecting ion dopant concentrations priorto annealing which cannot be performed with an electrical measurementtechnique.

In a manner analogous to that used to detect implanted ions, many othertypes of microscopic impurities or defects can also be detected by theapparatus of the subject invention.

It will be noted that the above referenced U.S. Pat. No. 4,636,088 isdirected to similar investigations based on the generation of thermalwaves. It has now been found that when a semiconductor material is beingtested, thermal waves do not have to be generated but, rather,investigations can be carried out by studying changing plasma density atthe surface of the sample. When materials other than semiconductors areto be evaluated, such as, metals and dielectrics, periodic heating willstill be necessary and analysis of the thermal wave patterns isrequired.

As set forth above, apparatus 20, is specifically designed formicroscopic residue and defect detection and evaluation of dopantconcentrations. Accordingly, apparatus 20 includes elements unnecessaryto perform the more general aspects of applicant's new and improvedmethod wherein electron-hole plasma interactions are studied. Forexample, information about a semiconductor on a more macroscopic scalewill not require the use of a microscopic objective for focusing theenergy and probe beams. Other elements which are illustrated in theapparatus of FIG. 1 which are not necessary for performing the basicsteps of the subject invention will be discussed in the text.

In FIG. 1, there is illustrated a semiconductor test sample 22 whichrests on a movable stage 24. Stage 24 is movable in two axes as shown byarrows A and B, enabling the sample to be rastered with respect to theenergy and probe beams. By this arrangement, two-dimensional mapping ofthe surface and subsurface characteristics of the semiconductor can bereadily achieved. Movable stages are well-known in the prior art andneed not be described in detail herein.

As discussed above, sample 22 is a semiconductor. By definition, asemiconductor will have a plurality of electrons in a valence band whichcan be excited across a band gap to a conduction band creatingelectron-hole pairs. In the literature, a plurality of theseelectron-hole pairs are called a plasma. In accordance with the subjectinvention, a means must be provided for exciting electrons in a mannerto bridge the band gap and generate an electron-hole plasma. In theillustrated embodiment, the energy is supplied by a laser 26. The energymay be supplied through any source of electromagnetic radiation or evenparticle beams, such as an electron beam. The operation of the subjectinvention also requires that this energy source be periodic.Accordingly, the beam 28 which is emitted from laser 26 is chopped by amodulator 30. The chopping frequency of the modulator 30 is controlledby a processor 32 which also handles the controls of the detectionportion of the device, discussed below.

As discussed above, the frequency of the modulator which chops the laserbeam determines whether plasma waves will be generated. This thresholdfrequency is a function of the time it takes for the electron-hole pairsto recombine in the semiconductor. As can be appreciated, if therecombination time is shorter than the time of each laser pulse, anywave action will die out. However, if the plasma state exists for a timelonger than the period of the modulation frequency, a wave-likephenomenon will be observed. A mathematical discussion of thecharacteristics of this wave-like phenomenon is disclosed in theattached Appendix. The generation of waves is necessary if it is desiredto perform some types of analyses involving depth profiling which areanalogous to thermal wave detection systems. In the situation wheredepth information is not required, it is not a necessity that plasmawaves be generated. However, in practice, frequencies are often chosenwhich do in fact result in the production of plasma waves. In operation,it has been found that chopping frequencies on the order of one MHz to100 MHz are most often utilized.

The frequency modulated laser beam 28 is directed to the surface of thesemiconductor. As shown in FIG. 1, the beam 28 passes through a dichroicmirror 34 and a microscopic objective 36. The arrangement of thedichroic mirror and microscopic objective facilitate the focusing of theenergy beam and the probe beam to microscopic spots on the surface ofthe sample. This arrangement is desirable where microscopic informationis desired. In a preferred embodiment, laser 26 is an argon ion laserand the dichroic mirror is transparent to wavelengths emitted by such alaser. The dichroic mirror will reflect radiation emitted from a heliumneon laser 40 which is used as the radiation probe.

As set forth in the subject invention, the energy level of laser 26 mustbe sufficient to excite electrons from the valence band to theconduction band of the semiconductor. This particular energy level is afunction of the type of semiconductor which is being analyzed. Theenergy level of the laser beam can be chosen to match this band gapenergy. If the laser beam has a higher energy than necessary, electronswill be excited above the band gap and then rapidly drop down to thelowest energy level of the conduction band. In the latter case, heatwill immediately be generated and thermal waves will also be induced. Itshould be noted that when the electrons eventually drop from theconduction band down to the valence band, that is, when the electronsand holes recombine, additional heat may be released. Where additionalheat is released in this latter step, its effect on the generation ofthermal waves may have to be considered, particularly if the plasma hasnot diffused a significant distance.

In the situation where the laser energy is sufficient to generateelectron-hole pairs, a plasma will be created which diffuses through thesemiconductor. The plasma will affect the index of refraction of thesemiconductor. More importantly, surface and subsurface features in thesemiconductor will affect the movement or diffusion of the plasma andthereby alter its local density and resultant effects on the changes tothe index of refraction.

In accordance with the subject invention, the changes in the index ofrefraction on the surface of the sample are monitored using a radiationprobe. As illustrated in FIG. 1, the radiation probe is provided by ahelium neon laser 40 which emits a beam 42 that is focused on thesurface of the sample using dichroic mirror 34 and microscopic objective36. As noted above, the dichroic mirror will reflect helium neonradiation. In order for the radiation probe to "see" the effects of theplasma, it must be directed within the periodically excited area. Thisarea is given by equation (1), cited above.

Where microscopic analysis is desired, further focusing through the useof a microscopic objective is desired. Typically, mirror 34 will also bemoveable to facilitate positioning.

As is well-known, a beam of radiation has both intensity and phasecharacteristics. When the beam is reflected off a sample surface wherethe index of refraction is changing, the probe beam will experience bothchanges in intensity (because the reflectivity of the sample ischanging) and changes in phase. The changes in reflectivity of thesample is given by the following equation:

    ΔR=(dR/dN)ΔN

where N is the plasma density and dR/dN is the plasma densitycoefficient of reflectivity.

As can be seen from the above equation, if one can measure changes inreflectivity, one can monitor periodic changes in plasma density whichare, in turn, a function of the surface and subsurface characteristicsof the sample. The apparatus in FIG. 1 is designed to measure thesechanges in reflectivity. More particularly, probe beam 42 is passedthrough a polarizing splitter 44. The polarizing splitter is oriented ina manner such as to let the coherent light 42 emanating from laser 40 topass freely therethrough. The splitter will, however, deflect all lightwhose phase has been rotated through 90° relative to beam 42. The reasonfor this arrangement will become apparent below.

Radiation probe beam 42 is then passed through a 1/4-waveplate 46.Waveplate 46 functions to rotate the phase of the probe beam by 45°. Ascan be appreciated, on the return path of the beam, the waveplate willrotate the phase of the beam another 45° so that when it reachessplitter 44, the phase of the beam will have been rotated a total of 90°from the incoming orientation. By this arrangement, the splitter 44 willreflect the retro-reflected light beam up to a photodetector 50, asdiscussed in more detail below.

After the probe beam passes through the 1/4-waveplate 46, it isdeflected downward by dichroic mirror 34. In the preferred embodiment ofthe subject invention, the energy beam 28 and the probe beam 42 arealigned in such a manner that they are directed in a coincident mannerdown through lens 36 and focused at essentially the same spot on thesurface of the sample. By focusing the probe beam and the energy beam atthe same spot, the maximum signal output can be achieved.

As the probe beam is reflected off the surface of the sample, itinteracts with the sample at its surface. The refractive index of thesample undergoes periodic changes as the plasma density changesperiodically. The probe beam essentially "sees" the changes of the indexof refraction induced by the density changes of the plasma such that theintensity and phase of the probe beam will be altered.

The probe beam is then reflected back up to the dichroic mirror where itis, in turn, reflected back along the incoming path and through the1/4-waveplate 46. As discussed above, waveplate 46 rotates the phase ofthe probe beam by another 45° such that when the beam reaches splitter44 it phase has been rotated 90° with respect to the original beam.Accordingly, the splitter will deflect the retroreflected probe beamupwardly towards photodetector 50.

Since intensity variations of a radiation beam are to be detected, astandard photodetector may be employed as a sensing mechanism. Theintensity variations which are measured are then supplied as an outputsignal to processor 32 for deriving information relating to the surfaceand subsurface conditions of the sample. The processor will analyzechanges which are in phase with the periodic changes of the energy beam28. In practice, the apparatus of the subject invention has proved to bea highly sensitive indicator of the presence of residues and otherdefects, as well as levels of ion dopant concentrations insemiconductors.

This analysis is relatively straightforward in that the sample willtypically be rastered with respect to the beams and the changing outputsignals which are in phase with the energy beam can be plotted toindicate variations in dopant or defect concentrations or the presenceof residues. These output signals can be compared to predeterminedreflectivity measurements made on a known reference sample. The latterinformation can be stored in the processor and compared to give relativeinformation concerning the tested sample.

In this analysis, there is no necessity to ensure that plasma waves begenerated. However, if one is interested in doing more sophisticatedanalysis of layer thickness or material composition as a function ofdepth, information can be derived by studying the interaction of plasmawaves with the subsurface features. These plasma waves are highly dampedsuch that they travel only one or two wavelengths before becoming tooweak to detect. However, the plasma waves will interact with variousmicroscopic features in the sample in a manner that is mathematicallyequivalent to scattering and reflection of conventional propagatingwaves. Any features on or beneath the surface of the sample that haveelectronic characteristics different from their surroundings willreflect and scatter the plasma waves and thus alter the diffusion of theplasma waves.

In order to ensure that plasma waves are generated, the frequency of themodulator must be set such that the modulation period is less than andpreferably much less than the recombination time of the plasma. In atypical semiconductor sample of silicon, the recombination time is onthe order of 10⁻⁵ seconds. Therefore, it is desirable to set themodulation frequency of the energy beam to be on the order of 1 MHz.

When plasma waves are generated, the diffusivity of the plasma is givenby equation (3) above. As noted, in this situation, changes in beammodulation frequency will affect the plasma diffusion length. Thus, byvarying the frequency of the modulator, one can derive additionalinformation to calculate either layer thicknesses or compositionalvariables as a function of depth. The introductory mathematics for aplasma wave analysis is set forth below.

Calculations and measurements using these plasma waves are directlyanalogous to that of critically damped thermal waves. A number ofapproaches for deriving additional information using thermal waves isset forth in U.S. Pat. No. 4,513,384, cited above. In the latterapplication, sophisticated mathematical modeling is disclosed to providein-depth analysis of surface and subsurface characteristics. Lesssophisticated analysis is also possible in an on-line situation wheremeasurements, taken on a known reference sample, while the frequency ofthe modulation of the energy beam is varied, are stored in theprocessor. These measurements can then be compared with a test sample toprovide relative measurements and information regarding surface andsubsurface features. This approach can also be used in the method of thesubject invention.

As discussed above, the change in the index of refraction of the sample22 induced by the changing plasma density, will also alter the phase ofthe incoming probe beam. FIG. 2 shows an alternate apparatus, inschematic form, for measuring the phase changes in the probe beam 42. InFIG. 2, only the probe beam portions of the subject apparatus areillustrated with like numbers being used to designate like components.

As illustrated in FIG. 2, one method of detecting phase changes of anelectromagnetic beam is through the use of an interferometry technique.Interferometers for measuring phase changes of lasers are well-known inthe art. Furthermore, an interferometer has been used to measure thermalwaves in an analogous manner. (See "Photo Displacement Imaging" Ameri etal., Photoacoustic Spectroscopy Meeting, Technical Digest, Paper THA6-1Optical Society of America, 1981.)

Some theories exist as to the mechanism by which the phase of the probebeam will be shifted when the index of refraction of a material ischanged. One theory relates to the changing of the path length of thebeam prior to its being reflected. Where the index of refraction ischanging periodically, the phase shift of the probe beam will vary in asinusoidal manner. These phase shifts can be detected with aninterferometer. It should be clear that an understanding of the theexact mechanism which causes the phase shift in the beam is notnecessary to carry out the objects of the subject invention. Rather, itis only necessary that this phase shift exists and can be measured.Furthermore, it is also important that this phase shift is a measure ofthe periodic changes in plasma density which is, in turn, dependent uponthe surface and subsurface features of the sample.

As illustrated in FIG. 2, a laser is provided having a beam 42 which ispassed through a partially transmissive mirror 60. A portion 42a of thebeam will pass through the mirror and travel along a path through thepolarizing splitter 44 and 1/4-waveplate 46 in a manner identical toFIG. 1. Another portion of the beam 42b will also be reflected upwardlyby the partially transmissive mirror 60 to an angled mirror 62. Thisportion 42b of the beam will be used as a reference for detecting thephase shift of the probe beam.

As in the first embodiment, the probe beam 42a is retroreflected off thesurface of the sample back pass through the 1/4-waveplate and directedupwardly by the polarizing splitter 44 to photodetector 50. Beam 42awill then pass directly through a partially transmissive mirror 64.Mirror 64 will also reflect the reference portion 42b of the probe beamupwardly into the photodetector 50.

At this point, the two portions of the radiation beam 42 have beenrecombined. As is well-known in the art, when two coherent beams oflight are combined that are not in phase, interference patterns willdevelop. These interference patterns take the form of train of intensityvariations which can be detected by the photodetector. In thisapplication, since the phase shift of the probe beam 42a is constantlychanging, the interference patterns will be constantly changing. Theseperiodic intensity changes, brought about by the interference of thebeams, are analyzed by the processor to yield information about theplasma density variations in the sample. An analysis can then be made ina manner described above with regard to FIG. 1.

The phase changes are given by a formula analogous to the intensitychange formula (4) as follows:

    ΔΦ=(dΦ/dN)ΔN

where Φ is the phase and N is the plasma density.

Similar to the embodiment shown in FIG. 1, the probe beam 44a must bedirected into the area that has been periodically excited by the energybeam. In addition, microscopic information can be obtained by focusingthe beams through a microscopic objective 36.

Another measurement technique can also be used to monitor the phasechanges in the probe. This alternative approach is analogous to thethermal wave detection technique disclosed in U.S. Pat. Nos. 4,521,118and 4,522,510, cited above. In the latter techniques, the periodicangular deflections of the reflected probe beam are measured to give anindication of thermal wave activity. In the subject apparatus, the probebeam will also undergo periodic angular deflections in response to theplasma wave activity because of the radial variations in the plasmadensity.

A measurement system which can detect these periodic angular deflectionscan be provided by modifying the apparatus shown in FIG. 1. The elementswhich require modifications are shown in FIG. 3. Similar to FIG. 1, aperiodic energy beam 28 and a probe beam 42 are directed to the surfaceof the sample 22. However, and as shown in FIG. 3, the changes in theprobe beam are detected by a split or bi-cell photodetector 50a.Photodetector 50a has two quadrants, each of which gives a separatemeasure of the intensity of the beam. When the beam fluctuates acrossthe surface of the detector, each side of the detector will experiencechanges in intensity which can be correlated by the processor to measurethe extent of the beam deflections. This form of detector is describedin more detail in the applications cited above.

As set forth in the latter two patent applications, in order to maximizethe signal in a deflection type technique, the probe beam 42 must bespaced from, but focused close to, the incoming energy beam 28. Thisarrangement is shown in FIG. 3 where microscope objective 36 is used tofocus both beams. In this case, the mirrors are arranged such that thetwo beams are parallel but noncoaxial.

In summary, there has been provided a new and improved apparatus forevaluating surface and subsurface conditions in a semiconductor sample.In accordance with the subject invention, a periodic excitation sourceis provided for supplying energy to the sample surface sufficient tocreate an electron-hole plasma. The diffusing plasma functions to changethe index of refraction of the surface of the sample. The diffusion ofthe plasma is, in turn, a function of the local sample characteristicsand thus the plasma induced changes in the index of refraction are alsoa function of these local sample characteristics. The changing index ofrefraction can be measured using a radiation probe which is reflectedoff the surface of the sample. Changes in the radiation probe are thenmonitored to evaluate the sample.

Having described the apparatus for carrying out the subject invention isdetail, our present understanding of the mathematics of plasma waveswill be set forth, As discussed above, it is well known that theabsorption of an intensity modulated energy beam (e.g., electron orlaser) results in a modulated temperature profile within the heatedmaterial having the properties of a critically damped propagating wave,that is a thermal wave (A. Rosencwaig, Photoacoustics and PhotoacousticSpectroscopy, (Wiley, N.Y., 1980)). Mathematically, the thermal wave issimilar to a conventional wave and being dependent upon the thermalproperties of the medium for its propagation, it undergoes reflectionand refraction at thermal boundaries, and a diffraction from the edgesof thermal features. Being critically damped, the thermal wave has theadded feature of becoming negligibly small at distances beyond one ortwo thermal wave lengths from its source. However, by selecting theappropriate modulation frequencies of the source beam and consequentlythe appropriate thermal wavelengths, the near-surface region within amaterial can be probed with thermal waves, while maintaining highspatial resolution.

Recently, a number of applications of thermal waves in semiconductormaterials have been demonstrated (A. Rosencwaig, Science 218, 223(1982); W. B. Jackson and N.M. Amer. Phys. Rev. B25, 5559 (1982); A.Rosencwaig, J. Opsal, and D. L. Willenborg, Appl. Phys. Lett. 43, 166(1983); J. Opsal, A. Rosencwaig, and D. L. Willenborg, Appl. Opt. 22,3169 (1983); M. A. Olmstead and N.M. Amer. Phys. Rev. Lett. 52, 1148(1984); W. L. Smith, J. Opsal, A. Rosencwaig, J. B. Stimmell, J. C.Allison and A. S. Bhandia, J. Vac. Sci. Technol. B2, 710 (1984)) and inthe process of some of these investigations we have discovered anotherkind of critically damped wave, in this case a variation in thephoto-induced plasma density which can be treated in an analogousfashion to the thermal wave.

By way of introduction, let us consider what happens when a laser beamis incident on a semiconductor. If the energy per photon, E, exceeds theband gap energy, E_(g), then electrons will be excited from the valenceband to an energy, E-E_(g) above the conduction band edge. Thesephoto-excited carriers, will, within a relatively short time (10⁻¹³sec), give up a portion of their energy (E-E_(g)) to lattice throughnonradiative transitions to the unoccupied states near the bottom of theconduction band. After a much longer time (10⁻³ -10⁻⁸ sec) thesephoto-excited carriers will recombine with holes in the valence band.Prior to this recombination, there thus exists a plasma of electrons andholes whose spatial density is governed by diffusion in a manneranalogous to the flow of heat from a thermal source. Thus, if anincident laser beam is intensity-modulated, we would expect to observe,in addition to the thermal wave, a modulated plasma density whosespatial profile is that of a critically damped wave, i.e., a plasmawave.

In order to estimate the significance of this plasma wave, considerfirst the heat equation in one dimension,

    ∂.sup.2 T∂x.sup.2 -(ρc/κ)∂t=-Q.sub.o /κ        (5)

which, assuming a sinusoidal time dependence, e^(i)ωt, leads to theequation for thermal waves,

    d.sup.2 T/dx.sup.2 +q.sup.2 T=-Q.sub.o /κ            (6)

where q is a thermal wave vector defined by ##EQU2## κ is the thermalconductivity, and Q_(o) is the heat source for which we have assumed asinusoidal time dependence of the form e^(-i)ωt. In Eq. (7), ρ isdensity, C is the specific heat and μ_(t) is the thermal diffusionlength. If we now assume that the thermal source is localized at thesurface, x=0, of a semi-infinite material, then we have the solution,

    T(x)=T.sub.o e.sup.iqx,                                    (8)

where T_(o) is the temperature at the surface,

    T.sub.o =iQ.sub.o /qκ.                               (9)

For the plasma wave we now have the analogous

plasma wave equation,

    d.sup. N/dx.sup.2 +p.sup.2 N=-P.sub.o /D,                  (10)

where p is the plasma wave vector defined by

    p=(1+i)(ωτ+i)/2Dτ).sup.1/2,                  (11)

D is the ambipolar diffusion coefficient, and P_(o) is the plasma sourceterm. Also, in Eq. (11), τ is the recombination time. The one essentialdifference between the plasma wave problem and the thermal wave problemis the existence of the recombination time. In the limit ωτ<<1, theplasma wave loses its wavelike properties becoming a purely diffusivephenomenon with a plasma diffusion length l_(p) =(Dτ)^(1/2). However, inSi (S. M. Sze, Physics of Semiconductor Devices, Wiley Interscience,N.Y., 1969, chapt. 2) and for modulation frequencies in the MHz regime,we have ωτ>1 (in many cases ωτ>>1), and the plasma density is thengoverned primarily by lossless diffusion analogous to the thermal wave.In this limit it is meaningful to introduce an additional diffusionlength, the plasma wave diffusion length, μ_(p) =(2D/ω)^(1/2), which wesee is formally identical to the thermal wave diffusion length byreplacing the thermal diffusivity, κ/ρC, in Eq. (7) by the ambipolardiffusion coefficient, D. As in the thermal wave problem, if we assumethat all of the plasma is created at x=0, then we have for the plasmadensity,

    N(x)=N.sub.o e.sup.ipx                                     (12)

where N_(o) is the plasma density at x=0,

    N.sub.o =iP.sub.o /pD                                      (13)

If we now let Q denote the total energy flux absorbed in the sample thenin terms of Q we have, Q_(o) =((E-E_(g))/E)) Q+E_(g) N/τ and P_(o) =Q/E.If the plasma diffusion length, l_(p) =(Dτ)^(1/2), is much longer thanthe thermal diffusion length μ_(t), which is often the case forintrinsic Si, then Q_(o) =((E=E_(g))/E)Q. That is since l_(p) >>μ_(t),the energy in the plasma, (E_(g) E) Q, which is given up to the latticeas heat when the electrons and holes recombine nonradiatively, is nowspread over a distance much greater than the thermal diffusion lengthand its contribution to the temperature profile (thermal wave) istherefore negligible.

As an example, let's consider a beam of 2.4 eV photons with an absorbedintensity 10⁵ W/cm² in Si as might be obtained from a 5mW Ar⁺ laser beamat 514-nm that is focused to a 1 micron radius. Using the literatureparameters for intrinsic Si (Sze, supra); ρ=2.33 gm/cm³, C=0.703 J/gm-°C, κ=1.42 W/cm-° C, D= 20 cm² /sec, τ>10 μ sec, and E_(g=) 1.1 eV, wehave for the magnitudes of the surface temperature and surface plasmadensity oscillations at a 1 MHz modulation frequency,

    T.sub.o =14 ° C.

and

    N.sub.o =2.3×10.sup.19 /cm.sup.3.

These temperature and plasma oscillations, that is, the thermal andplasma waves, are of course, of practical significance only is they canbe detected and measured. One effect of an increasing temperature in asemiconductor such as Si is a narrowing of the band gap andconsequently, an increase in the optical reflectivity. Measurements overa wide range of energy and temperature yield a coefficient of thermalreflectance in Si (H. A. Weakliem and D. Redfield, J. Appl. Phys. 50,1491 (1979)) of (1/R_(o))(dR/dT)=1.5× 10⁻⁴ /° C. where R_(o) is thesurface reflectance in the absence of any temperature change. Thus, wewould expect to observe an amplitude modulation in the opticalreflectivity of Si, ΔR/R_(o) ≃2.1×10⁻³ arising from the surfacetemperature modulation. In a recent paper on thermal wave effects inmetals, (A. Rosencwaig, J. Opsal, W. L. Smith, and D. L. Willenborg,Appl. Phys. Lett., Vol 46 page 1013, 1985) we have shown that suchthermal wave induced changes in optical reflectance are readilymeasured. Here, however, we also must consider the effects of the plasmaon the optical reflectivity. This is basically a Drude effect with anegative coefficient of reflectance which has been discussed (A. M.Bonch-Bruevich, V. P. Kovalev, G. S. Romanov, Ya. A. Imas, and M. N.Libenson, Sov. Phys. Tech. Phys. Rev. Lett. 32, 1120 (1974); J. M. Liu,H. Kurz, and N. Bloembergen, Appl. Phys. Lett. 41, 643 (1982)) andfemtosecond (C. V. Shank, R. Yen, and C. Hirlimann, Phys. Rev. Lett 50,454 (1983)) duration. For a He-Ne laser probe beam of wavelength, λ=633nm, only the real part of the index of refraction in Si is significant.Thus, for the plasma coefficient of reflectance we have (1/R_(o))(dR/dN)=-2λ² /πn(n² -1)mc², where n=3.9 is the index of refraction,e=4.8×10.sup. esu is the electron's charge, M=0.15m_(o) is the effectivemass in terms of the bare electron mass m_(o) =9.1×10⁻²⁸ gm, andc=3.0×10¹⁰ cm/sec is the speed of light. That is, (1R_(o))(dR/dN)=-9.6×10⁻²³ cm³, which implies a plasma wave induced modulation in theoptical reflectivity of ΔR/R_(o) ≃-2.2×10⁻³. According to this modelthen, one would expect a net modulation of the optical reflectivity ofSi of ˜10⁻⁴. Our experimental results on Si are in agreement with thisobservation. That is, the reflectance signal we measure is about anorder of magnitude smaller than predicted on the basis of there onlybeing a thermal wave present in the material.

To be more realistic, 3- dimensional effects should be included. Asdescribed by Opsal et al, (J. Opsal, A. Rosencwaig, and D. L.Willenborg, Appl. Opt. 22, 3169 (1983)) this may be accomplished by atreatment based on a linear superposition of 1- dimensional solutions.

In summary we have introduced a new kind of critically damped wave whichwe call the plasma wave, present whenever there is a periodic excitationof the plasma density in semiconductors. We have furthermoredemonstrated its significance through measurements of the modulatedreflectance on two samples of p-type Si. Analagous to the use of thermalwaves as a probe of local variations in thermal features, these plasmawaves are of great practical significance in that they can be used todetect changes in material properties which affect their propagation.Such features include: lattice defects, ion implantation damage, cracks,voids and delaminations, to name a few. In some cases, for example inion implanted regions, we expect plasma waves to be affected much moresignificantly than thermal waves. In general, we believe plasma waveswill provide a complementary capability to the thermal waves as atechnique for characterizing semiconductor materials.

While the subject invention has been described with reference topreferred embodiments, it is to be understood that various other changesand modification could be made therein by one skilled in the art withoutvarying from the scope and spirit of the subject invention as defined bythe claims appended hereto.
 1. An apparatus for evaluating surface andsubsurface conditions in a semiconductor sample comprising:a periodicexcitation source for supplying energy to the surface of the samplesufficient to create an electron-hole plasma having a density sufficientto cause changes in the optical reflectivity of the sample; a probe foremitting a beam of radiation of a fixed wavelength shorter than thewavelength corresponding to the band-gap energy of the sample; means fordirecting the radiation probe beam within a portion of the surface ofthe sample which has been periodically excited in a manner such thatsaid probe beam is reflected; means for monitoring the modulatedintensity changes in said reflected probe beam resulting from thevariations in the optical reflectivity of the sample due principally tothe presence of the electron-hole plasma; and means for processing themeasured intensity changes of the reflected probe beam to evaluate thesample.
 2. An apparatus as recited in claim 1 wherein said probe beam isdirected toward the center of the area on the surface of the samplewhich has been the periodically excited.
 3. An apparatus as recited inclaim 1 wherein the monitoring means includes a photodetector.
 4. Anapparatus as recited in claim 1 wherein the processing means function tocompare the monitored changes of the probe beam with predeterminedchanges of the probe beam associated with a known reference samplewhereby variations in surface and subsurface conditions in the samplecan be monitored.
 5. An apparatus as recited in claim 1 furtherincluding a means for rastering the sample with respect to both theexcitation source and radiation probe such that a two dimensionalevaluation can be made.
 6. An apparatus as recited in claim 1 whereinsaid probe beam is defined by a laser.
 7. An apparatus as recited inclaim 1 further including a means for controlling the modulationfrequency (ω) of the excitation source.
 8. An apparatus as recited inclaim 7 wherein the modulation frequency (ω) of said excitation sourceis set such that the period between cycles is less than the time (τ)required for electron-hole pairs to recombine (ωτ greater than 1)whereby critically damped plasma waves are generated.
 9. An apparatus asrecited in claim 8 wherein changes in the probe beam are monitored asthe modulation frequency (ω) of the energy source is varied to provideinformation about subsurface characteristics of the sample.
 10. Anapparatus as recited in claim 9 wherein monitored changes in the probebeam are compared with predetermined changes of the probe beamassociated with a known reference sample whereby variations insubsurface conditions of the sample can be monitored.
 11. A method forevaluating surface and subsurface conditions in a semiconductor samplecomprising the steps of:supplying periodic energy to the surface of thesample to excite electrons and create an electron-hole plasma having adensity sufficient to cause changes in the optical reflectivity of thesample; directing a radiation probe beam on a portion of the area on thesurface of the sample which has been periodically excited in a mannersuch that the probe beam is reflected, said probe beam being of a fixedwavelength and shorter than the wavelength corresponding to the band-gapenergy of the sample; monitoring changes in the modulated intensity ofthe reflected probe beam resulting from the changes in the opticalreflectivity of the sample due principally to the presence of theelectron-hole plasma; and processing the monitored intensity changes ofthe probe beam to evaluate the sample.
 12. A method as recited in claim11 wherein said radiation probe is directed towards the center of thearea on the surface of the sample which has been periodically excited.13. A method as recited in claim 11 wherein during said processing step,the changes of said probe beam are compared with predetermined changesof the probe beam associated with a known reference sample, wherebyvariations in the surface and subsurface conditions in the sample can bemonitored.
 14. A method as recited in claim 11 further including thestep of setting the modulation frequency (ω) of the periodic energysource such that the period between cycles is less than the time (τ)required for electron-hole pairs to recombine whereby critically dampedplasma waves are generated.
 15. A method as recited in claim 14 furtherincluding the step of monitoring changes in the probe beam as themodulation frequency (ω) of the energy source is varied to provideinformation about subsurface characteristics of the sample.
 16. A methodas recited in claim 15 further including the step of comparing themeasured changes in the probe beam to predetermine changes of the probebeam associated with a known reference sample, whereby variations in thesubsurface conditions of the sample can be monitored.
 17. A method asrecited in claim 11 further including the step of rastering the samplewith respect to the probe and energy beams to permit a two-dimensionalanalysis.
 18. An apparatus for evaluating surface and subsurfaceconditions in a semiconductor sample comprising:an intensity modulatedlaser energy beam for supplying energy to the surface of the samplesufficient to create an electron-hole plasma having a density sufficientto cause changes in the optical reflectivity of the sample; a laserprobe for emitting a beam of radiation of a fixed wavelength shorterthan the wavelength corresponding to the band-gap energy of the sample;means for directing the radiation probe beam within a portion of thesurface of the sample which has been periodically excited in a mannersuch that said probe beam is reflected; means for monitoring variationsin the modulated intensity of said reflected probe beam resulting fromchanges in the optical reflectivity of the sample due principally to thepresence of the electron-hole plasma; and means for processing themeasured intensity changes of the reflected probe beam to evaluate thesample.
 19. An apparatus as recited in claim 18 wherein the probe beamis directed towards the center of the area on the surface of the samplewhich has been periodically excited.
 20. An apparatus as recited inclaim 18 wherein the monitoring means includes a photodetector.
 21. Anapparatus as recited in claim 18 wherein the processing means functionsto compare the monitored changes of the probe beam with predeterminedchanges of the probe beam associated with a known reference sample,whereby variations in the surface and subsurface conditions in thesample can be monitored.
 22. An apparatus as recited in claim 18 furtherincluding a means for rastering the sample with respect to both theexcitation source and the radiation probe such that a two-dimensionalevaluation can be made.
 23. An apparatus as recited in claim 18 furtherincluding a means for controlling the modulation frequency (ω) of theexcitation source.
 24. An apparatus as recited in claim 23, wherein themodulation frequency (ω) of said excitation source is set such that theperiod between cycles is less than the time (τ) required forelectron-hole pairs to recombine whereby critically damped plasma wavesare generated.
 25. An apparatus as recited in claim 24, wherein changesin the probe beam are monitored as the modulation frequency (ω) of thelaser energy beam is varied to provide information about the surface andsubsurface characteristics of the sample.
 26. An apparatus as recited inclaim 25, wherein monitored changes in the probe beam are compared withpredetermined changes of the probe beam associated with a knownreference sample, whereby variations in subsurface conditions of thesample can be monitored.
 27. An apparatus as recited in claim 18 furtherincluding a focusing lens and wherein both said intensity modulatedlaser energy beam and said probe laser beam are focused normal to thesurface of the sample through said lens.
 28. A method for evaluatingsurface and subsurface conditions in a semiconductor sample comprisingthe steps of:directing a periodic laser beam at the surface of thesample having an input energy sufficient to excite electrons and createan electron-hole plasma having a density sufficient to cause changes inthe optical reflectivity of the sample; directing a laser probe beam ona portion of the area on the surface of the sample which has beenperiodically excited in a manner such that the probe beam is reflected,said probe beam being of a fixed wavelength and shorter than thewavelength corresponding to the band-gap energy of the sample;monitoring changes in the modulated intensity of the reflected probebeam resulting from the changes in the optical reflectivity of thesample due principally to the presence of the electron-hole plasma; andprocessing the monitored intensity changes of the probe beam to evaluatethe sample.
 29. A method as recited in claim 28 wherein said radiationprobe is directed towards the center of the area on the surface of thesample which has been periodically excited.
 30. A method as recited inclaim 28 wherein during said processing step, the changes of said probebeam are compared with predetermined changes of the probe beamassociated with a known reference sample, whereby variations in thesurface and subsurface conditions in the sample can be monitored.
 31. Amethod as recited in claim 28 further including the step of setting themodulation frequency (ω) of the periodic laser beam such that the periodbetween cycles is less than the time (τ) required for electron-holepairs to recombine whereby critically damped plasma waves are generated.32. A method as recited in claim 31 further including the step ofmonitoring changes in the probe beam as the modulation frequency (ω) ofthe periodic laser beam is varied to provide information aboutsubsurface characteristics of the sample.
 33. A method as recited inclaim 28 further including the step of rastering the sample with respectto the probe and energy beams to permit a two-dimensional analysis. 34.A method as recited in claim 28 further including the step of focusingthe periodic laser beam and the probe laser beam normal to the surfaceof the sample through the same optical element.